1. Field of the Invention
The present invention relates to a device for detecting rotation of a rotating object by magneto-resistance elements that are located in a biasing magnetic field.
2. Description of the Related Art
U.S. Pat. No. 6,452,381 B1 or JP-A-11-237256, which is a counterpart application of the U.S. patent, discloses such a rotation detecting device. This device includes a hollow biasing permanent magnet and a sensor chip on which a pair of magneto-resistance elements is formed. The sensor chip is inserted into the inside hollow portion of the permanent magnet so as to confront gear teeth formed at the periphery of a rotating object.
The magneto-resistance sensor is constituted of a pair of magneto-resistance elements formed on a sensor chip in such a shape of reversed V that the distance between the pair of sensor elements becomes shorter as they approach the gear teeth. Therefore, the resistance of one of the sensor elements changes differently from the other when the rotating object rotates. Because the sensor elements form into a half bridge circuit having a middle point, an electric output signal that changes as the rotation angle of the rotating object changes can be obtained from the middle point of the half bridge circuit. In other words, the potential of the middle point cyclically changes in response to the concave-convex portions of the gear teeth.
The resistance of the magneto-resistance sensor element changes when the magnetic vector of the biasing magnetic field provided by the permanent magnets changes. However, if the sensor chip is not correctly positioned relative to the gear teeth, the magnitude of change in the magnetic vector may become insufficient for the magneto-resistance sensor to provide sensor signals.